restricted value passed to a sub question

[ToddAndMargo via perl6-users]

Hi All,

I am cooking up something where I want top pass a value to a sub, but I 
want to restrict what those values are.

For instance, things like

   AbortRetryIgnore
   CancelRetryContinue
   Help
   YesNo
   Maybe

And so on and so forth.

If the wrong value is passed, I want the checker to crash
the sub.

What is the best way of going about this?

Many thanks,
-T